Calcium salt for calcium batteries

ABSTRACT

Disclosed is a calcium salt, Ca(HMDS)2, where HMDS is the hexamethyldisilazide anion (also known as bis(trimethylsilyl)amide), enables high current densities and high coulombic efficiency for calcium metal deposition and dissolution. These properties facilitate the use of this salt in batteries based on calcium metal. In addition, the salt is significant for batteries based on metal anodes, which have higher specific energies than batteries based on intercalation anodes, such as LiC6. In particular, a calcium based rechargeable battery includes Ca(HMDS)2 salt and at least one solvent, the solvent suitable for calcium battery cycling. The at least one solvent can be diethyl ether, diisopropylether, methyl t-butyl ether (MTBE), 1,3-dioxane, 1,4-dioxane, tetrahydrofuran (THE), tetrahydropyran, glyme, diglyme, triglyme or tetraglyme, or any mixture thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 17/716,226 filed onApr. 8, 2022, now U.S. Patent Application Publication No. 2022-0367914entitled “Calcium Salt for Calcium Batteries.” U.S. Ser. No. 17/716,226is a continuation of U.S. Ser. No. 16/694,950 filed on Nov. 25, 2019,now U.S. Pat. No. 11,342,589 entitled “Calcium Salt for CalciumBatteries.” U.S. Ser. No. 16/694,950 claims priority to and the benefitof U.S. Provisional Application No. 62/772,868 filed Nov. 29, 2018 andentitled “Calcium Salt for Calcium Batteries”. The disclosure of theforegoing applications is incorporated herein by reference in itsentirety, including but not limited to those portions that specificallyappear hereinafter, but except for any subject matter disclaimers ordisavowals, and except to the extent that the incorporated material isinconsistent with the express disclosure herein, in which case thelanguage in this disclosure shall control.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under W911NF-17-1-0466awarded by the Army Research Office. The government has certain rightsin the invention.

TECHNICAL FIELD

The present disclosure relates to electrochemistry of calcium ions, andin particular, to a calcium salt that enables high current densities andhigh coulombic efficiency for calcium metal deposition and dissolution.

BACKGROUND

Across the globe, scientist are interested in what is coming afterlithium batteries. The rechargeable batteries have become the dominantpower source in any number of consumer electronics, from phones to cars.It has been suggested that the global lithium ion battery market willhit 100 billion by 2024.

However, that growth comes with several challenges. For example, thereare limited sources suitable for large-scale lithium extraction. Inaddition, lithium extraction results in extreme environmental impacts,such as heavy water consumption. Thus, there is a continued need forreplacement of lithium-based batteries.

Calcium has the potential to act as a replacement. For example, it isalready used in lead acid batteries, which are often seen in automotivestarter motors. Calcium is the fifth most abundant element in theEarth's crust and the third most abundant metal, with equal geographicalresource distribution around the globe. Calcium also lacks lithium'stroublesome trait of catching fire. The challenge for the adoption ofcalcium has been finding a workable electrolyte. Within batteries, anelectrolyte is a medium that allows the battery to move ions from oneplace to another. For a battery to work, it needs to transfer ions fromits cathode to its anode and back. Electrolytes come in many varieties,including soluble salts, acids, or other bases in liquid, gelled, anddry formats.

SUMMARY

Disclosed herein is a method of providing a battery, comprising:providing an anode; providing a cathode; calcium salt, comprising a Ca²⁺cation and two hydrocarbon substituted disilazide anions; and at leastone solvent, suitable for calcium battery cycling.

In embodiments, the hydrocarbon substituted disilazide anions compriseor consist of a hexamethyldisilazide anion or a derivative thereof. Inembodiments, the hydrocarbon substituted disilazide anions comprise orconsist of —N(SiMe3)(SiMe2tBu), where tBu refers to a tertiary butylgroup, —N(SiMe3)(SiPh2tBu), where Ph refers to a phenyl group,—N(SiMePh2)2, —N(SiMe3)(SiPh3), or —N(SiTvle3)(Mes), where Mes refers toa mesityl group, or N(SiMe3)(Dipp), where Dipp refers to2,6-di-isopropylaniline, in embodiments, the at least one solventincludes or consists of diethyl ether, diisopropyl ether, methyl t-butylether (MTBE), 1,3-dioxane, 1,4-dioxane, tetrahydrofuran (THF),tetrahydropyran, glyme, diglyme, triglyme, tetraglyme, a secondary ortertiary amine or any mixture thereof.

In embodiments, the calcium salt dissociates, in solution with at leastone solvent, into Ca²⁺ and HMDS⁻ ions or derivatives thereof such that aconcentration of Ca²⁺ (in all its speciated forms) is greater than orequal to 1.0M, In embodiments, the concentration of Ca²⁺ enables currentdensities for calcium metal deposition and dissolution >30 mA/cm² at 25mVis. In embodiments, a fraction of THF in the electrolyte that iscoordinated in a coordination sphere of the Ca²⁺ is sufficiently high toprevent deleterious reactions with a calcium metal anode. Inembodiments, the concentration of free THE is one of: 10 mole %, such asbelow 10 mole %, including below 9 mole %, below 8 mole %, below 7 mole%, below 6 mole %, below 5 mole %, below 4 mole below 3 mole %, below 2mole %. below 1 mole %, below 0.1 mole %, below 0.01 mole %, for examplebetween 10 mole % and 0.01 mole %, or between 1 mole % and 0.01 mole %,between 1 mole % and 0.1 mole %, between 0.1 mole % and 0.01 mole %,such as 9 mole %, 8 mole %, 7 mole %, 6 mole %, 5 mole %, 4 mole %, 3mole %, 2 mole %, 1 mole %, 0.5 mole %, 0.1 mole %, 0.05 mole %, or 0.01mole % of a total amount of solvent in the electrolyte. In embodiments,at least one solvent is THF and the concentration of calcium salt in theTI-IF is between 1 M and 2.47 M.

In embodiments, the calcium salt is dissolved in the solvent at aconcentration greater than a mole fraction of 0,065, In embodiments, thecalcium salt is provided as a liquid electrolyte, which is dissolved inat least one solvent. In embodiments, the calcium salt is dissolved in apolymer gel electrolyte in at least one solvent, and mixed with asuitable polymer matrix.

In embodiments, the cathode includes a redox active material capable ofCa²⁺ insertion. In embodiments, the redox active material is at leastone of: V₂O₅, Mn₂O₄, or MnO₂.

In embodiments, the cathode includes a material used for Na⁺ insertion.In embodiments, the material used of Na⁺ insertion is at least one ofNaMnFe₂(PO₄)₃, NaVO₃; or Na₂FeP₂O₇.

The foregoing and other features of the disclosure will become moreapparent from the following detailed description, which proceeds withreferences to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the following description and accompanying drawings:

FIG. 1 illustrates both single cycle and multiple cycle data forCa(HMDS)₂ dissolved in THF at a concentration of 1 M. In accordance withvarious embodiments (Pt(w), Pt(c), Ca(ref), 25 mV/s). Ca(HMDS)₂ wassynthesized and rigorously dried THF (Na benzophenone) was used.Hexamethyldisilazide has a pKa reported to be 26, so it will driveCaO(s) formation in the electrolyte, consuming water. Cyclic voltammetryfor Ca electrodeposition is shown at (upper) right for a single scan and15 repetitive scans shown at (lower) right. Electrodeposition appearsrelatively reversible and very high current densities, >30 mA cm⁻² at 25mV s⁻¹ were observed. FIG. 1 also provides the chemical structure ofHMDS.

FIG. 2 is a plot of average Coulombic efficiency versus mole fractionfor Ca(HMDS)₂ dissolved in THF, in accordance with various embodiments.Transition at mole fraction=0.06-0.07. Low coulombic efficiency at lowCa(HMDS)₂ mole fraction was observed. Plot shows a transition incoulombic efficiency, with markedly higher values at higher [Ca(HMDS)₂].It is believed that at very high mole fractions of Ca(HMDS)₂,essentially all TI-IF is coordinated by Ca²⁺(i.e. “free” [THF] isapproximately zero). Infrared spectroscopy was used to examine anychanges in the electrolyte over this concentration range.

FIG. 3 illustrates FTIR of THF at increasing Ca(HMDS)₂ mole fraction. Inparticular, FTIR results show the 910 cm⁻¹ region of THF that allowsdifferentiation of free and coordinated THE. As demonstrated, THE ringmode shows decreasing “free” THF (910 cm⁻¹) and increasing coordinatedTHF (875 cm⁻¹) as mole fraction of Ca(HMDS)₂ increases. Sigmoidal fitalso showed inflection point at mole fraction of 0,066, consistent withthe CE results. Saturation at mole fraction 0.16 is consistent with a“solvate” IL, [Ca(HMDS)(THF)₆][HMDS].

FIG. 4 illustrates 17 mM Ca(TFSI)₂ in BMP TFSI ionic liquid whereincoulombic efficiency starts at less than 50% and current densitydegrades with cycling and Ca²⁺/Ca showed only limited reversibility inBMP (Pt(w), Pt(c), Pt(ref), 50 mV/s).

FIG. 5 illustrates Ca²⁺/Ca in BMP TFSI ionic liquid+ diglyme and BH₄. Asillustrated, diglyme and free BH₄ did not have much effect. Best resultfor coulombic efficiency was Ca 65% per cycle. Studies illustrated thatstrategies that were effective for Mg²⁺/Mg are not effective forCa²⁺/Ca.

FIG. 6 illustrates cycling as a function of [Ca(HMDS)₂]/THF. Very highsolubility, was observed, enabling high current densities (>30 mA cm⁻²)and studies over a wide range of concentrations. As concentrationincreased, current density was observed to increase and then decrease.Concentrated solutions are extremely viscous and higher viscosity can bea signature of a “solvate” ionic liquid.

FIG. 7 illustrates cycling current densities for Ca²⁺/Ca inCa(BMDS)₂/THF. Cycling demonstrates high current densities over multiplecycles. 30 mA cm′ is greater than 2× higher than any previously reportedfor Ca and much higher (>10×) than Mg. This electrolyte hascharacteristics of a “solvate” ionic liquid or a “solvent-in-salt”.Ca/Ca²⁺ shows superior cycling behavior in the solvate ionic liquidcompared to lower concentrations.

FIG. 8 is a graph illustrating the effect of amine concentration onreversible deposition.

FIG. 9 is a graph illustrating the effect of Ca(HMDS)₂ concentration onreversible deposition.

FIG. 10 illustrates the effect of cyclic charging and discharging.

DETAILED DESCRIPTION

The following description is of various exemplary embodiments only, andis not intended to limit the scope, applicability or configuration ofthe present disclosure in any way. Rather, the following description isintended to provide a convenient illustration for implementing variousembodiments including the best mode. As will become apparent, variouschanges may be made in the function and arrangement of the elementsdescribed in these embodiments without departing from principles of thepresent disclosure.

For the sake of brevity, conventional techniques and components may notbe described in detail herein. Furthermore, the connecting lines shownin various figures contained herein are intended to represent exemplaryfunctional relationships and/or physical couplings between variouselements. It should be noted that many alternative or additionalfunctional relationships or physical connections may be present inexemplary systems and/or components thereof.

Unless otherwise explained, all technical and scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which this disclosure belongs. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of this disclosure, suitable methods andmaterials are described below. In addition, the materials, methods, andexamples are illustrative only and not intended to be limiting.

The following explanations of terms and methods are provided to betterdescribe the present compounds, compositions and methods, and to guidethose of ordinary skill in the art in the practice of the presentdisclosure. It is also to be understood that the terminology used in thedisclosure is for the purpose of describing particular embodiments andexamples only and is not intended to be limiting.

As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise.

As used herein, the term “and/or” refers to and encompasses any and allpossible combinations of one or more of the associated listed items, aswell as the lack of combinations when interpreted in the alternative(“or”).

As used herein, “one or more” or at least one can mean one, two, three,four, five, six, seven, eight, nine, ten or more, up to any number.

Lithium battery technology is generally well known. The next frontier ismultielectron battery systems based on metals such as, for example, Mg²⁺and Ca²⁺. Over the past several years considerable effort has been spenton Mg²⁺ battery science while little work has been done on calciumbatteries. However, it is disclosed herein that calcium batteries areadvantageous and attractive for a number of reasons.

For example, on a volumetric basis, calcium is just as attractive as Li(2072 mA hr cm⁻³; and 2062 mA hr cm⁻³, respectively). Part of the issueis that calcium metal is even more reducing than Mg, with a similarreduction potential to Li. This potent reducing power drives electrolytedecomposition, leading to poor reversibility. Attempts to reversiblydeposit calcium metal have, for the most part, failed. One recent paperdescribes how Ca(BH₄)₂ may be dissolved in tetrahydrofuran (THF), andthat this electrolyte allows reasonably reversible deposition anddissolution of calcium metal. However, the paper further indicates thatthe calcium metal reacts with THF, producing a surface film containingCaH₂ (calcium hydride), which thus fouls the surface. The CaH₂ surfacefilm thus interferes with achieving high coulombic efficiency, becauseits formation consumes calcium metal (Plating and stripping calcium inan organic electrolyte, by: Wang, Da; Ciao, Xiangwen; Chen, Yuhui; etal. NATURE MATERIALS Volume: 17 Issue: 1, Pages: 16-20. Published:January 2018). Thus additional calcium based batteries are needed.

Disclosed herein is a calcium based rechargeable battery. The calciumbased rechargeable battery includes a calcium salt that comprises orconsists of a Ca²⁺ cation and two hydrocarbon substituted disilazideanions. The calcium based rechargeable battery further incudes at leastone solvent, suitable for calcium battery cycling; for example a solventthat can solvate the Ca²⁺ cation and the two hydrocarbon substituteddisilazide anions.

In embodiments, the calcium salt is a Ca²⁺ cation and two hydrocarbonsubstituted disilazide anions, for example a methyl substituteddisilazide anion, such as a hexamethyldisilazide anion (also known asbis(trimethylsilyl)amide) or a derivative thereof, such as, such as⁻N(SiMe₃)(SiMe₂ ^(t)Bu), where ^(t)Bu refers to a tertiary butyl group,⁻N(SiMe₃)(SiPh₂ ^(t)Bu), where Ph refers to a phenyl group,⁻N(SiMePh₂)₂, ⁻N(SiMe₃)(SiPh₃), or ⁻N(SiMe₃)(Mes), where Mes refers to amesityl group or N(SiMe₃)(Dipp), where Dipp refers to2,6-di-isopropylaniline.

In embodiments, a calcium salt, having substituted disilazide anions,for example a methyl substituted disilazide anion, such as ahexamethyldisilazide anion (also known as bis(trimethylsilyl)amide) or aderivative thereof, enables high current densities and high coulombicefficiency for calcium metal deposition and dissolution. Theseproperties facilitate the use of this salt in batteries based on calciummetal. In addition, the salt is significant for batteries based on metalanodes, which have higher specific energies than batteries based onintercalation anodes, such as LiC₆, the anode used in most rechargeableLi batteries today.

It is noted that, in embodiments, the calcium salt is preferably highlysoluble (such as >1 M) in the solvent, or solvent system used, therebyenabling very high concentrations of the Ca²⁺-containing solutionspecies that are reduced at the battery's anode during charging. Inembodiments, the high concentration of the Ca²⁺-containing solutionenables high current densities to be used. In embodiments, highsolubility of the calcium salt enables the production of very high localconcentrations of soluble Ca²⁺-containing species during batterydischarge. On the other hand, low solubility (such as <10 mM) generallylimits the current densities that can be used during charging anddischarging.

In embodiments, the calcium salt, having substituted disilazide anions,for example a methyl substituted disilazide anion, such as ahexamethyldisilazide anion (also known as bis(trimethylsilyl)amide) or aderivative thereof enables even high concentrations of Ca²⁺ to bedissolved in various solvents suitable for calcium battery cycling. Inembodiments, the calcium salt dissociates, in solution with at least onesolvent, into Ca²⁺ and HMDS⁻ ions or derivatives thereof such that thetotal concentration of Ca²⁺ in all its spectated forms is greater thanor equal to 1.0 M. In embodiments, the calcium salt is dissolved in thesolvent at a concentration greater than a mole fraction of 0.065. Inembodiments, this high concentration has two beneficial effects: first,it enables very high current densities for calcium metal deposition anddissolution (e.g., >30 m A/cm² at 25 mV/s) in an electrolyte comprisedof calcium salt, having substituted disilazide anions, for example amethyl substituted disilazide anion, such as a hexamethyldisilazideanion (also known as bis(trimethylsilyl)amide) or a derivative thereofand any of a variety of solvents, including but not limited totetrahydrofuran (THF). Second, at sufficiently high concentrations ofthe calcium salt, having substituted disilazide anions in THF, there isseen an unexpected increase in the coulombic efficiency for the calciummetal deposition/dissolution cycle. This increase may be attributed tothe complexation of the solvent in the coordination sphere of the Ca²⁺ion, such that there is only a very small concentration of “free” (i.e.uncoordinated) THF. Thus, it disclosed herein that this complexationprevents deleterious reactions of THF with the calcium metal anode,thereby enabling high coulombic efficiency. In this embodiment, theamount of free THF in the electrolyte is reduced, because much of it iscoordinated in the Ca²⁺ coordination sphere. The Ca²⁺-THF complex ismore stable toward calcium metal than free THF, In embodiments, afraction of THF in the electrolyte that is coordinated in a coordinationsphere of the Ca²⁺ is sufficiently high to prevent deleterious reactionswith a calcium metal anode. In embodiments, the concentration of freeTHF is one of: 10 mole %, such as below 10 mole %, including below 9mole %, below 8 mole %, below 7 mole %, below 6 mole below 5 mole %,below 4 mole below 3 mole %, below 2 mole %, below 1 mole %, below 0.1mole %, below 0.01 mole %, for example, between 10 mole % and 0.01 mole%, or between 1 mole % and 0.01 mole %, between 1 mole % and 0.1 mole %,between 0.1 mole % and 0.01 mole %, such as 9 mole %, 8 mole %, 7 mole%, 6 mole %, 5 mole %, 4 mole %, 3 mole %, 2 mole %, 1 mole %, 0.5 mole%, 0.1 mole %, 0.05 mole or 0.01 mole % of a total amount of solvent inthe electrolyte. In embodiments, the solvent is THF and theconcentration of calcium salt in the THF is between 1 M and 2.47 M.

It is noted that the calcium salt, having substituted disilazide anions,is not available commercially, and must be synthesized. Thus, heretoforeit has not yet been explored as a salt for use in calcium-basedbatteries. In fact, the only salt that has been successfully studied forreversible calcium metal deposition/dissolution is Ca(BH₄)₂. Whileprevious studies using this salt reported concentrations up to 1.5 M,yielding current densities <15 mA/cm² at 25 mV/s, in stark contrast, theinventors' preliminary work with Ca(HMDS), salt has yielded currentdensities that are better than this figure by more than a factor of two.Further, borohydride salts, such as Ca(BH₄)₂, are not suited for use inbatteries, due to possible instability and release of hydrogen gas (H₂)during unwanted decomposition. Thus, substituted disilazide anions, suchas HMDS, represent the only salts now known that provide highconcentrations of Ca²⁺, thereby enabling current densities sufficientlyhigh to enable reasonable battery performance.

As a result, there are no known suitable salts that can serve asalternatives to achieve high solubility of Ca²⁺ in solvents suitable forrechargeable calcium batteries. An important feature for battery use ishow high a coulombic efficiency for calcium metal deposition/dissolutionusing calcium salt, having substituted disilazide anions may beobtained. At present, values as high as 80% have been observed, andgoing forward, it is expected that substantially higher values may beachieved by focusing on salt and solvent purity. Further, increasedcoulombic efficiency may be realized by exploring the electrochemicalbehavior of calcium salts having substituted di silazide anions, forexample, a methyl-substituted disilazide anion such ashexamethyldisilazide (also known as bis(trimethylsilyl)amide), or aderivative thereof in other solvents, such as, for example, aminesolvents, such as dimethylethylamine.

In embodiments, a calcium based rechargeable battery includes aCa(HMDS)₂ salt and at least one solvent, suitable for calcium batterycycling. In embodiments, the at least one solvent includes THEAlternatively, for example, the at least one solvent may include one ormore of: diethyl ether, diisopropylether, methyl t-butyl ether (MTBE),1,3-dioxane, 1,4-dioxane, tetrahydropyran, glyme, diglyme, triglyme,tetraglyme or an amine based solvent, such as N,N-dimethylethylamine(DMEA), triethylamine (TEA), and tetramethylethylenediamine (TMEDA). Inembodiments, a calcium based rechargeable battery does not includeCa(BH₄)₂.

In embodiments, the calcium salt, having substituted disilazide anions,for example a methyl substituted disilazide anion, such as ahexamethyldisilazide anion (also known as bis(trimethylsilyl)amide) or aderivative thereof dissociates in solution with the solvent into Ca²⁺and HMDS⁺ ions, the concentration of Ca²⁺ being greater than or equal to3.0 M.

In embodiments, batteries made using the calcium salt, havingsubstituted disilazide anions, for example a methyl substituteddisilazide anion, such as a hexamethyldisilazide anion (also known asbis(trimethylsilyl)amide) or a derivative thereof may include either aliquid electrolyte comprised of this salt dissolved in any of thesolvents listed above, or, for example, a polymer gel electrolytecomprised of this salt in a dissolved state in any of the solventslisted above, and mixed with a suitable polymer matrix (e.g., a polymergel electrolyte). The cathode of the battery may use a redox activematerial capable of Ca²⁺ insertion, such as, for example, V₂O₅, Mn₂O₄,MnO₂, or any of a number of cathode materials previously used for Na′insertion. In embodiments, this class of materials would be suitablebased on the similar ionic sizes of Na⁺ and Ca²⁺. Examples of suchmaterials include NaMnFe₂(PO₄)₃, NaVO₃, Na₂FeP₂O₇, or the like.

FIGS. 1-7 described how at high concentrations of Ca(HMDS)₂, theelectrolyte has characteristics of a “solvate” ionic liquid (IL). It ishere noted that an ionic liquid is simply a low melting salt. Thetypical definition is that it is in its liquid state at temperaturesbelow 100 degrees C. In contrast, a solvate ionic liquid is one in whichessentially all of the solvent is engaged in direct, inner spherebonding with the metal ion. Under such circumstances, the free solventcontent is vanishingly small, such as less than 10 mole %. Thus, inembodiments, an electrolyte system may be provided that includes calciumsalt, having substituted disilazide anions, for example a methylsubstituted disilazide anion, such as a hexamethyldisilazide anion (alsoknown as bis(trimethylsilyl)amide) or a derivative thereof dissolved invarious solvents as noted above, under conditions where the result is asolvate ionic liquid. These include, for example, compositions in whichthe calcium salt, having substituted disilazide anions, for example amethyl substituted disilazide anion, such as a hexamethyldisilazideanion (also known as bis(trimethylsilyl)amide) or a derivative thereofhas a mole fraction higher than 0.14. As described in the Example below,this value is based upon a sigmoidal fit of the FUR spectroscopic data,which shows a limiting value achieved somewhere between 0.14 and 0.16mole fraction of Ca(HMDS)₂. It is contemplated that methods in additionto spectroscopic tools may be used to define where the solvatecomposition occurs.

The following example is provided to illustrate particular features ofcertain embodiments. However, the particular features described belowshould not be construed as limitations on the scope of the disclosure,but rather as examples from which equivalents will be recognized bythose of ordinary skill in the art.

EXAMPLES Example 1

The inventors synthesized a Ca(HMDS)₂ salt using known techniques. Whilethis salt has been described in the general area of synthesis of calciumcompounds, there is no reference to its use in electrochemical settings.It is further noted that the results below reflect the behavior ofrelatively impure materials that may have some amount of deleteriouslyreactive impurities, as the salt used by the inventors was not totallypure. The results of these studies are shown in FIGS. 1-7 .

The inventors have shown that the Ca(HMDS)₂ salt may be dissolved in THFup to concentrations of at least 2.47 M. FIG. 1 shows single cycle andmultiple cycle data for Ca(HMDS)₂ dissolved in TI-IF at a concentrationof 1 M. As can be seen in FIG. 1 , on a negative-going scan, metaldeposition begins near −0.8 V versus a calcium reference electrode. Onthe return scan, metal stripping (metal oxidative dissolution) initiatesnear −0.7 V, and the metal stripping charge shows good behavior. Theratio of the metal stripping charge to the charge for metal depositiongives the Coulombic efficiency (CE). In various implementations, theinventors have achieved CE values near 80%. Interestingly, it is notedthat very low CE values were seen at low concentrations of Ca(HMDS)₂ inTHF, i.e. near 0.75 M. However, much higher values were seen forconcentrations of 1 M and above. This is shown in FIG. 2 , whichpresents a plot of CE versus mole fraction of Ca(HMDS)₂ in theelectrolyte. As shown, a remarkable increase in CE near a mole fractionof 0.065 occurs.

Additionally, infrared spectroscopy was used to examine the THFstretching vibrations in an attempt to understand the remarkable andunexpected increase in CE with concentration/mole fraction, describedabove. In this analysis, the difference in vibrational frequencies forTHE that is free in the electrolyte and THE that is coordinated with itsoxygen atom to the Ca²⁺ ions in the electrolyte was taken advantage of.As shown in FIG. 3 , the fraction of free THE decreases substantially asthe Ca(HMDS)₂ mole fraction is increased. This change occurs over thesame range of mole fraction as does the increase in CE. Thus, theincreased CE is attributed to the reduced availability of THE fordeleterious reactions with the calcium metal surface. These deleteriousreactions may otherwise lead to calcium metal consumption and poorreversibility for the metal deposition and dissolution processes.

It is important to note that the high concentrations to which theCa(HMDS)₂ salt can be dissolved in THE, or other solvents, enables thisbehavior.

Thus, a significant increase in the CE of calcium metal deposition anddissolution occurs at very high concentrations (or, it may be said, atvery high mole fractions) of Ca(HMDS)₂ salt in THE. It is believed thistrend represents a general trend and thus is expected to be seen inother solvents into which Ca(HMDS)₂ can be dissolved. Thus, Ca(HMDS)₂salt may be beneficially used in calcium batteries.

In summary, speciation of metal ions in ionic liquids is key indirecting the behavior of their redox processes. TFSI⁻ is clearly a badactor under reducing conditions (especially with reactive radicals) andshould likely be avoided for such applications. Further, BH₄ ⁻ scavengeswater, but is much more effective when not coordinated. HMDS⁻ includes anew anion type for multivalent systems that can control water andenhance metal ion solubility, giving access to concentration rangeswhere solvate ILs can be formed. HMDS⁻ enables extremely high Ca²⁺solubility in a range of solvents, allowing for quite good redoxbehavior for Ca/Ca²⁺.

Example 2

New Electrolytes for Calcium Battery Systems Based on Amine Solvents

Based on the successful use of PEO compounds to keep the reactiveodd-electron species Mg away from possible reaction partners, theinventors explored new complexation systems based on amines. Theobjective was to suppress reaction of Ca²⁺ with oxygen-containingcompounds by providing N-donor ligands. The inventors examined aminecompounds both as additives and as neat solvents. The compounds examinedincluded N,N-dimethyethylamine (DMEA), Triethylamine (TEA), andTetramethylethylenediamine (TMEDA).

FIG. 8 shows the effect of amine additives: Ca(HMDS)₂/Toluene/DMEA.Ca(HMDS)₂ dissolves at high concentration in toluene; but noelectrochemistry (suggests fully ion-paired). The stoichiometricaddition of DMEA turns on the electrochemistry, giving reasonablyreversible deposition. The effect saturates at 4 mol DMEA/1 mol Ca²⁺.This is consistent with speciation as [Ca(HMDS)(DMEA)₄]⁺.

FIG. 9 shows a new electrolyte: Ca(HMDS)₂/dimethylethylamine(DMEA)/Ca(HMDS)₂ also dissolves at high concentrations in neat DMEA.Using this system chemically reversible reductive deposition can beseen. Solutions with high [Ca(HMDS)₂] are very viscous, like theCa(HMDS)₂/THF system. This electrolyte gives reasonable reversibilityfor Ca anode electrochemistry.

FIG. 10 shows Ca(HMDS)₂/dimethylethylamine (DMEA)+TEA. Ca(HMDS)₂dissolves at high concentrations in DMEA/TEA mixtures, Bestreversibility so far is with 50 mol % TEA (based on Ca).

While the principles of this disclosure have been shown in variousembodiments, many modifications of structure, arrangements, proportions,the elements, materials and components, used in practice, which areparticularly adapted for a specific environment and operatingrequirements may be used without departing from the principles and scopeof this disclosure. These and other changes or modifications areintended to be included within the scope of the present disclosure.

The present disclosure has been described with reference to variousembodiments. However, one of ordinary skill in the art appreciates thatvarious modifications and changes can be made without departing from thescope of the present disclosure. Accordingly, the specification is to beregarded in an illustrative rather than a restrictive sense; and allsuch modifications are intended to be included within the scope of thepresent disclosure.

Likewise, benefits, other advantages, and solutions to problems havebeen described above with regard to various embodiments. However,benefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element.

As used herein, the terms “comprises,” “comprising,” or any othervariation thereof, are intended to cover a non-exclusive inclusion, suchthat a process, method, article, or apparatus that comprises a list ofelements does not include only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. Also, as used herein, the terms “coupled,”“coupling,” or any other variation thereof, are intended to cover aphysical connection, an electrical connection, a magnetic connection, anoptical connection, a communicative connection, a functional connection,and/or any other connection. When language similar to “at least one ofA, B, or C” or “at least one of A, B, and C” is used in thespecification or claims, the phrase is intended to mean any of thefollowing: (1) at least one of A; (2) at least one of B; (3) at leastone of C; (4) at least one of A and at least one of B; (5) at least oneof B and at least one of C; (6) at least one of A and at least one of C;or (7) at least one of A, at least one of B, and at least one of C.

What is claimed is:
 1. A calcium based rechargeable battery, comprising: a calcium salt, comprising a Ca²⁺ cation and two hydrocarbon substituted disilazide anions; at least one solvent, suitable for calcium battery cycling; and an amine additive.
 2. The battery of claim 1, wherein the amine additive comprises N,N-dimethylethylamine (DMEA), triethylamine (TEA), or tetramethylethylenediamine (TMEDA).
 3. The battery of claim 1, wherein the amine additive comprises DMEA, and wherein the at least one solvent comprises toluene.
 4. The battery of claim 3, wherein a stoichiometric ratio between the calcium salt and the amine additive is about 4 mol amine additive to 1 mol calcium salt.
 5. The battery of claim 1, wherein the amine additive comprises DMEA, and wherein the at least one solvent comprises DMEA.
 6. The battery of claim 1, wherein the amine additive comprises DMEA and TEA, and wherein the at least one solvent comprises DMEA and TEA.
 7. The battery of claim 6, wherein a stoichiometric ratio between the calcium salt and the TEA is about 50 mol % TEA.
 8. The battery of claim 1, wherein the hydrocarbon substituted disilazide anions comprise a hexamethyldisilazide anion or a derivative thereof.
 9. The battery of claim 1, wherein the hydrocarbon substituted disilazide anions comprise ⁻N(SiMe₃)(SiMe₂ ^(t)Bu), where ^(t)Bu refers to a tertiary butyl group, N(SiMe₃)(SiPh₂ ^(t)Bu), where Ph refers to a phenyl group, ⁻N(SiMePh₂)₂, ⁻N(SiMe₃)(SiPh₃), or ⁻N(SiMe₃)(Mes), where Mes refers to a mesityl group, or N(SiMe₃)(Dipp), where Dipp refers to 2,6-di-isopropylaniline.
 10. A method of providing a battery, comprising: providing an anode; providing a cathode; providing a calcium salt comprising a Ca²⁺ cation and two hydrocarbon substituted disilazide anions; dissolving the calcium salt in at least one solvent, the at least one solvent suitable for calcium battery cycling; and providing an amine additive that suppresses reaction of the Ca²⁺ cation with oxygen-containing compounds.
 11. The method of claim 10, wherein the amine additive supplies N-donor ligands to suppress reaction of the Ca²⁺ cation with oxygen-containing compounds.
 12. An ionic liquid, comprising: a calcium salt comprising a Ca²⁺ cation and two hydrocarbon substituted disilazide anions; at least one solvent, suitable for calcium battery cycling; and an amine additive.
 13. The ionic liquid of claim 12, wherein the amine additive comprises N,N-dimethylethylamine (DMEA), triethylamine (TEA), or tetramethylethylenediamine (TMEDA).
 14. The ionic liquid of claim 12, wherein the amine additive comprises DMEA, and wherein the at least one solvent comprises toluene.
 15. The ionic liquid of claim 14, wherein a stoichiometric ratio between the calcium salt and the amine additive is about 4 mol amine additive to 1 mol calcium salt. 